1. Field of the Invention
This invention relates to the field of reprographic papers used for copying architectural plans, engineering drawings and charts.
2. Description of Related Art
Reprographic papers, in particular blueprint and brownprint or sepia papers, have been used for many years by architects and engineers to make to copies of plans, drawings and charts. These papers, which are not based on silver halides, utilize a relatively simple technology which can be used in the field and does not required a darkroom.
The original blueprint papers were based on ferric salts and ferricyanides. Iron based papers, however, are relatively insensitive to light and require aqueous development.
The original sepia processes utilized iron salts together with another metal salt, in particular silver salts. Although silver salts are light sensitive, they were not used in sepia papers for their light sensitivity, but only as image formers.
In order to overcome the disadvantages of iron-based papers, diazo papers were developed in the 1920's and are still used today. Diazo papers are based upon the photosensitivity of diazo compounds RN.dbd.NX, where R is a hydrocarbon radical and X is any electronegative substituent. The diazo component is combined in the paper with a coupling component, and when the exposed paper is developed by exposure to ammonia gas, the diazo and coupling components form a colored diazo dye.
The first rapid developing diazo papers were disclosed in 1927 in U.S. Pat. No. 1,628,279, which utilized p-dimethyl- and p-diethyl-aminobenzenediazonium chloride sensitizers. This general technology is still used today in the use of 2,5-dialkoxy-4-morpholinobenzenediazonium compounds as sensitizers. After exposure to light, the paper is developed by exposure to the vapor of aqueous ammonia.
After development, these reprographic papers are frequently subjected to frequent and rough handling, and exposure to harsh outdoor environments, including bright sunlight. In the past, users of the papers would attempt to impart some degree of protection by taping the edges with one-inch wide masking tape, so that one half inch of tape would overlap the edge on both the front and back of the blueprint. This is of course a time consuming process, and makes the drawings difficult to handle; in time, the tape will yellow and become brittle.